Photoactive 2-nitro-4-azidophenyl groups will be attached to the non-histone chromosomal proteins HMG-1 and HMG-17, which will also be labelled with 125 I. The modified proteins will be introduced into He La cells and human lymphocytes by red cell mediated microinjection, and allowed to migrate into the nuclei and bind to the appropriate sites on the chromatin. The culture will then be irradiated with visible light to attach the proteins to interest of the chromatin covalently. After isolation of the chromatin, chemical and enzymatic techniques will be used to establish whether the HMG proteins are bound to core particles or the linker regions, whether they are associated with transcriptionally active portions of the genome, and which other proteins are found in close proximity to them. Studies will also be done to determine the turnover rates of the chromatin-bound HMG proteins and to establish which portions of the HMG molecule are involved in the interaction with chromatin.